1. Field of the Invention
This invention relates to an active noise attenuating device provided in a propagation path of noise for producing a sound having the same amplitude as that of the noise and a phase opposite to the noise, to cause a sound interference, thereby attenuating the noise.
2. Description of the Prior Art
An active noise attenuating device has recently been proposed for attenuating noise produced by an air conditioner and propagating along a draft duct thereof. The active noise attenuating device produces a sound having the same amplitude as that of the noise and a phase opposite to the noise to cause a sound interference in the draft duct, thereby actively attenuating the noise and reducing an amount of noise leaking out of the draft duct.
An active noise attenuating technique applied to the above-described device employs applied electronic techniques and particularly, an acoustic data processing circuit arrangement and acoustic interference. In this active noise attenuating technique, basically, a microphone is provided in the draft duct to detect the sound from a noise source, thereby converting the detected sound to a corresponding electrical signal. The electrical signal is processed into a signal by an operation unit. The signal is supplied to a loud speaker so that it produces an artificial sound having the same amplitude as of the noise and the phase opposite to the noise, at a control point and so that the artificial sound interferes with the noise at the control point.
An attenuation efficiency can be expected to amount to 10 dB or more in a low frequency band in the above-described device. Moreover, no pressure loss occurs in the above noise attenuation device. For example, when a concert hall is equipped with the above-described active noise attenuating device, noises produced from the draft ducts can be attenuated such that a better space can be provided for appreciation of music.
In employment of the active noise control in practice, characteristic variations due to aged deterioration of parts composing the signal system and due to an ambient temperature need to be coped with. For this purpose, an operational factor or acoustic transfer function of the operation unit is adjusted in accordance with variations in the noise attenuating performance of the device. More specifically, a monitoring microphone is provided for monitoring the noise attenuating effect of a loud speaker. Adaptive control means is also provided for controlling the operation unit. When the monitorial result is out of a predetermined allowable range, the adaptive control means changes the operational factor of the operation unit so that the monitorial result is within the allowable range. Consequently, the noise attenuation performance in the active noise control is maintained at its optimum in accordance with the characteristic variations. This control manner is referred to as "adaptive control."
FIG. 5 illustrates an example of the conventional active noise attenuating device as described above. A sound source microphone 2 for detecting noise, a loud speaker 3 producing an interference sound and a monitoring microphone are disposed along a noise propagation path in an air-conditioning draft duct 1. A detection signal generated by the microphone 2 is supplied via a low pass filter (LPF) 7 and an analog-to-digital (A/D) converter 8 to an input section of a finite impulse response (FIR) filter 6 serving as the operation unit in a control section 5 generating a control signal for producing the interference sound. The FIR filter 6 processes the detection signal from the microphone 2 by operation and generates a control signal, which signal is supplied to the loud speaker 3 via a digital-to-analog (D/A) converter 9, an LPF 10 and an amplifier 11. An adaptive filter 12 is provided for adjusting an operation factor of the FIR filter 6. The detection signal from the microphone 2 is supplied to the adaptive filter 12 via the LPF 7 and an A/D converter 8. Furthermore, a detection signal generated by the monitoring microphone 4 is supplied to the adaptive filter 12 via an LPF 13 and an A/D converter 14.
Generation of the control signal by the control section 5 will be described. The control section 5 processes the detection signal from the microphone 2 on the basis of the following characteristic: EQU G.sub.SO =G.sub.SA.multidot. G.sub.AO ( 1)
where G.sub.AO is an acoustic transfer characteristic between a point A indicative of the position of the loud speaker 3 and a point O indicative of the position of the monitoring microphone 4, G.sub.SO an acoustic transfer characteristic between a point S indicative of the sound source microphone 2 and the point O, and G.sub.SA an acoustic transfer characteristic between the point S and the point A. Then, a transfer characteristic G of the FIR filter 6 of the control section 5 needs to have an opposite phase with the acoustic transfer characteristic G.sub.SA between the points S and A. From the equation (1), the transfer characteristic G of the FIR filter 6 is obtained as follows: EQU G=-G.sub.SA =-G.sub.SO /G.sub.AO .multidot. (2)
Accordingly, the noise can be attenuated by the interference sound produced from the loud speaker 3 at the position of the monitoring microphone 4 when the transfer characteristic G of the FIR filter 6 is set at a value shown by the equation (2).
The signals from the microphones 2 and 4 are converted by the A/D converters 8 and 14 to digital signals respectively, which signals are supplied to the control section 5. These digital signals are processed by the control section 5. More specifically, high frequency components out of an objective noise frequency range are eliminated by the LPF 7 from the detection signal generated by the sound source microphone 2. The detection signal produced from LPF 7 is then sampled at a sampling frequency f and converted to a digital signal by the A/D converter 8. The sampling frequency f is set to a value twice as large as an upper limit frequency intended for noise attenuation or more so that a sampling theorem is satisfied.
In order that an interference sound having the same amplitude as of the noise and a phase opposite to it is produced, the FIR filter 6 processes the digitized detection signal indicative of the noise so that the amplitude and the phase of the detection signal are adjusted, thereby generating a control signal for production of the interference sound. The control signal is converted by the D/A converter 9 to an analog signal, which signal is supplied to LPF 10 eliminating higher harmonic alias components from the analog signal. The analog signal is then supplied via the amplifier 11 to the loud speaker 3. The interference sound produced from the loud speaker 3 interferes with the noise in the draft duct 1 to attenuate it.
The monitoring microphone 4 monitors the interference sound produced from the loud speaker 3 so that it is determined whether or not a sufficient attenuation effect is being achieved, thereby generating a detection signal. Based on the detection signal from the monitoring microphone 4, the adaptive filter 12 adjusts the operation factor of the FIR filter 6.
During the noise attenuation, the detection signal from the sound source microphone 2 is converted by the A/D converter 8 to the digital signal, which signal is supplied to both the FIR filter 6 and the adaptive filter 12. Furthermore, the digital signal is supplied to the adaptive filter 12 via the A/D converter 15 from the monitoring microphone 4. Based on these two digital signals, the adaptive filter 12 sets the operation factor of the FIR filter 6, for example, by a least-mean square (LMS) algorithm, so that the level of the signal generated by the monitoring microphone 4 is rendered the minimum or so that an amount of noise attenuated becomes the maximum. Thus, an adaptive control is performed so that the active noise control of the FIR filter 6 is usually executed efficiently.
In the above-described noise attenuation device, the system linearity is one of factors for determining the attenuation effect. A coherence function represented as a function of frequency is one of indexes of the system linearity. The attenuation effect of the system can be estimated by measuring the coherence function.
This coherence function serves to evaluate the system transfer function. Since an output is determined only by an input when the transmission system for a signal indicative of an measured object is linear and there is no noise contamination in the system. Accordingly, the coherence function takes the value of "1." On the other hand, the coherence function takes the value smaller than "1" when the signal transmission system is not linear or when there is some noise contamination in the system. FIG. 6 shows the relationship between changes in the coherence value and variations of an amount of noise attenuated. As understood from FIG. 6, the noise can be completely attenuated when the coherence value of the active noise attenuating device is "1" in a specified frequency range under the condition that the interference sound can be produced desirably. However, the noise can be attenuated only by 20 dB when the coherence value is reduced to 0.9. The reproducing performance of the loud speaker 3 reproducing the interference sound is one of factors reducing the coherence value or influencing the linearity of the system. More specifically, a lower limit of the frequency band of the sound to be reproduced by a loud speaker generally ranges 40 to 50 Hz. A wavelength of the reproduced sound exceeds the diameter of a cone composing the loud speaker when the frequency of the input signal is at 40 to 50 Hz or below. Although the cone of the loud speaker is caused to move back and forth to vibrate air in response to the input signal, a sound cannot be reproduced because the efficiency of converting the electrical signal to vibration is too low.
In the vibration characteristic of the cone of the loud speaker 3, it tends to vibrate with a large amplitude when the frequency of the signal input to the loud speaker 3 belongs to the above-described low frequency band and the amplitude of the signal is reduced as the frequency of the input signal is increased. Accordingly, the cone of the loud speaker is caused to unnecessarily move back and forth when an unreproducible low frequency signal is input to the loud speaker. Consequently, the vibration of the cone in response to a simultaneously input high frequency signal is prevented and the loud speaker cannot reproduce the sound with fine linearity. Thus, the characteristic of the loud speaker 3 is rendered nonlinear when the loud speaker 3 is supplied with the interference sound signal containing the unreproducible low frequency, so that the coherence value is reduced and an expected noise attenuation effect cannot be achieved.